Both Olympus E-PM1 and Pentax Q-S1 have 12.0 MP resolution sensors but Olympus E-PM1's sensor is Four Thirds (17.3 x 13 mm ) and Pentax Q-S1's sensor is 1/1.7" (7.44 x 5.58 mm ).
Since Olympus E-PM1's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Pentax Q-S1.
Below you can see the E-PM1 and Q-S1 sensor size comparison.
As seen above, Olympus E-PM1 has a 5.4x Larger sensor area than Pentax Q-S1. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.